1. Field of the Invention
The present invention relates to the field of compression ignition engines.
2. Prior Art
Compression ignition engines are well known in the prior art. While such engines can potentially operate on a wide range of liquid and gaseous fuels, commercially available compression ignition engines are limited to operation on diesel fuel and biodiesel fuels. Historically, diesel engines emitted substantial quantities of unburned hydrocarbons and NOX. Such emission levels are no longer considered acceptable. Accordingly, recent developments have been incorporated to clean up the exhaust of diesel engines so that the same are competitive with currently available gasoline engines. However, adoption of alternate fuels for compression ignition engines has heretofore not succeeded, not because compression ignition cannot be achieved but because compression ignition is very difficult to achieve, and when achieved, the pressure and temperature spike that results raises the temperature in the combustion chamber to well above that at which NOX forms. Also, very high mechanical compression ratios (the ratio of maximum to minimum combustion chamber volume) are usually required to obtain compression ignition for other fuels, making the design of such engines difficult. In particular, high mechanical compression ratios mean that the combustion chamber volume when the piston is at top dead center must be very small, and since that volume is spread over an area at least as large as the piston, the thickness of the volume in the combustion chamber when the piston is at top dead center is small, which among other things results in substantial heat transfer from the very hot gasses in the combustion chamber to the surfaces defining that volume, and further provides a large area for a given combustion chamber volume which can thermally quench and prevent combustion of a fuel/air mixture immediately adjacent that relatively large surface area.
Two fuels that have interesting possibilities for use in combustion ignition engines are ammonia and natural gas. Ammonia is of interest because it can be manufactured from other sources of energy, particularly non-polluting sources such as wind and solar, and when burned, merely exhausts nitrogen (assuming the temperature below which NOX will form is not exceeded) and steam which merely condenses to water vapor. Thus the products of combustion of ammonia are simply nitrogen, which already makes up approximately 80% of the atmosphere, and harmless water vapor. On the other hand, natural gas, while still a hydrocarbon, is of interest primarily because of its quantity and low cost, which therefore has the potential of substantially reducing the U.S. dependence on foreign oil.
Fuels like ammonia and natural gas have a combination of problems. First, the high or very high mechanical compression ratios required to obtain ignition, and second, the tendency of the combustion to exceed the temperatures at which NOX is formed once ignition is obtained. Also, for a gaseous fuel, injection of the gas into a combustion chamber in sufficient quantities for immediate ignition at the temperatures and pressures adequate for self ignition is near impossible. Consequently for gaseous fuels, the fuel needs to be mixed with the intake air, premixed so to speak, so control of the piston position (generally crankshaft angle for all except free piston engine embodiments) for ignition is very important, generally requiring a very versatile and controllable engine. Also even for liquid fuels, better mixing of the fuel and air is achievable to reduce combustion hot spots if the fuel and air are similarly premixed.
In U.S. Pat. No. 3,964,452, a spark ignition engine is disclosed that actually has a variable mechanical compression ratio. In particular, either a separate, spring loaded piston is provided in the engine head for each combustion chamber, or the engine piston itself is spring loaded so it can deflect downward when necessary. For both embodiments, once ignition is achieved and the pressure and temperature in the combustion chamber begin to spike, the spring loaded piston deflects, actually increasing the combustion chamber volume, which limits the pressure spike and most importantly the temperature spike.
The advantages of operating a compression ignition engine as an HCCI (homogeneous charge compression ignition) engine are well known in the prior art. In accordance with such operation, fuel is pre-mixed with air, either by injection of the fuel into the combustion chamber early in the compression stroke, or mixed with air in the intake manifold. This allows time for vaporization of liquid fuel, and for thorough mixing of the air and fuel, whether a liquid fuel or a gaseous fuel is being used. Consequently, ideally on ignition, combustion is uniform without the creation of hot spots, and combustion is complete because of the absence of fuel rich locations in the combustion chamber which do not thoroughly burn. Consequently, a compression ignition engine operating in an HCCI mode is particularly clean and highly efficient. The difficulty, however, that is commonly encountered in the prior art is that the amount of fuel (fuel/air ratio) that may be effectively used is quite limited, thereby limiting the power output of a particular engine to much less than the engine potentially could produce. The problem with adding more fuel to get more power from an engine operating as an HCCI engine is that all of the fuel that will be injected into the combustion chamber is already present in the combustion chamber at the time of ignition. Thus, unless the fuel/air ratio is kept relatively low, there will be a large spike in pressure and temperature, resulting in temperatures at which nitrous oxides are formed.
One approach for addressing this problem is disclosed in U.S. Pat. No. 6,910,459 entitled “HCCI Engine with Combustion-Tailoring Chamber”. In accordance with that patent, for each cylinder of the engine, an auxiliary combustion chamber and an inlet passage between the main combustion chamber and the auxiliary combustion chamber are formed in the engine head, with a control valve controlling communication between the main combustion chamber and the auxiliary chamber. Two embodiments are actually disclosed, though both use a conventional inlet (intake) valve operating system so that the inlet valve is driven between open and closed positions in a fixed relationship with the rotation of the crankshaft. Both embodiments also use a single auxiliary combustion chamber. The control valve, on the other hand, is electro-hydraulically controlled so as to allow variable timing with respect to crankshaft rotation.